Several methods of cooling, usually liquefying, a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form because it occupies a smaller volume and does not need to be stored at high pressures.
As an example of liquefying natural gas, the natural gas, comprising predominantly methane, enters an LNG plant at elevated pressures and is pre-treated to produce a purified feed steam suitable for liquefying at cryogenic temperatures. The purified gas is processed through a plurality of cooling stages using heat exchangers to progressively reduce its temperature until liquefaction is achieved.
Especially for long distance transportation, the liquefied natural gas can be carried in a sea-going vessel between, for example, an export terminal and an import terminal. On its return journey, the sea-going vessel can transport another liquefied gas such as liquid nitrogen, whose cold energy can then be used in the liquefaction of natural gas.
GB 1 596 330 relates to a process for the production of a liquefied natural gas on a sea-going vessel, where liquid nitrogen is passed through a heat exchanger situated on board the vessel to liquefy gaseous natural gas. All of the cold energy of the liquid nitrogen is used against one stream of natural gas, thus making the energy-matching of the liquefaction and the evaporation of the two streams difficult to balance.
DE 1 960 515, with reference to FIGS. 1 and 2 therein, discloses methods for liquefying a pressurized natural gas stream by heat exchanging against liquid nitrogen, wherein about two thirds of the gas is expanded in a turbine to a pressure of 1.1 ata and liquefied in a heat exchanger by heat exchanging against the liquid nitrogen which evaporates as a result. About one third of the gas is compressed to a high pressure of 200 ata with the aid of work released by the expansion of the about two thirds of the gas in the turbine, and expansion of the evaporated nitrogen stream in a turbine. The high-pressure natural gas is then cooled in a heat exchanger, depressurized to a pressure of 20 ata over a valve and further cooled and liquefied by heat exchanging against the vaporized and the expanded nitrogen.
FIG. 3 of DE 1 960 515 illustrates a method wherein about one third of the incoming natural gas is liquefied at pipeline pressure by heat exchange with the expanded nitrogen vapour and another refrigerant cycled in an additional refrigeration cycle, with for example a hydrocarbon mixed stream as the other refrigerant. This method needs the additional refrigeration cycle.
DE 1 960 515 thus presents different embodiments aiming at maximising the amount of natural gas that can be liquefied for each kg of liquid nitrogen, but a drawback of DE 1 960 515 is that the equipment count is rather high.